Gti wont rev, Help!

[doross]

My gti 1.8T wont rev past 5000rpm when its warmed up. It struggles just past 5000 in second gear but in third when the engine is under more load it only revs to 5000.
It feels like Im dragging a dirty great big anchor behind me.
Any ideas please what might be causing this.
Only mods is a forge dumpvalve and a performance air filter, both of which have been on the car for some time and the problem has only just developed.
 
Before this happened it was running like a dream. Help appreciated.

[charri42]

It sounds like the MAF sensor or a damaged CAT to me. Although a knackered CAT normally inhibits performance even more than that.

Someone who knows more will be along in a minute... Either way, its something 'external' to the engine and unlikely to be a major £Â£Ã‚£